Lithographic apparatus and device manufacturing method

ABSTRACT

Provided is a radiation distribution system for distributing the radiation from an illumination system to two or more patterning means, each for patterning beams of radiation, which are subsequently projected onto a substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Non-Provisional ApplicationNo. 10/779,823, filed Feb. 18, 2004, now U.S. Pat. No. 7,190,434, issuedMar. 13, 2007.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a lithographic apparatus and a devicemanufacturing method.

2. Related Art

A lithographic apparatus is a machine that applies a desired patternonto a target portion of a substrate. A lithographic apparatus can beused, for example, in the manufacture of integrated circuits (ICs), flatpanel displays and other devices involving fine structures. In aconventional lithographic apparatus, a patterning means, which isalternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern corresponding to an individual layer of theIC (or other device, such as a flat panel display), and this pattern canbe imaged onto a target portion (e.g., comprising part of, one orseveral dies) on a substrate (e.g., a silicon wafer or glass plate) thathas a layer of radiation-sensitive material (resist). Instead of a mask,the patterning means may comprise an array of individually controllableelements which serve to generate the circuit pattern.

In general, a single substrate will contain a network of adjacent targetportions that are successively exposed. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion in one pass, andso-called scanners, in which each target portion is irradiated byscanning the pattern through the projection beam in a given direction(the “scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction.

In a lithographic projection apparatus using arrays of individuallycontrollable elements, it is often necessary to use a plurality of sucharrays in order to expose the pattern on a substrate sufficientlyquickly that the though-put time for a substrate being exposed in theapparatus is economical. Furthermore, each array requires a relativelylarge amount of space around it for its support services such as data orcontrol lines required for setting the pattern on each array. It istherefore not appropriate to simply illuminate all of the arrayssimultaneously with a single illumination field.

What is needed, therefore, is an approach to provide an arrangement forsupplying radiation to each of the arrays of individually controllableelements that is economical and does not introduce additional sources ofproduction error in the pattern exposed on the substrate.

SUMMARY OF THE INVENTION

Consistent with the principles of the present invention as embodied andbroadly described herein, an apparatus includes an illumination systemfor supplying a projection beam of radiation, patterning means servingto the projection beam with a pattern in its cross-section, and secondpatterning means serving to impart a second beam of radiation suppliedby the illumination system with a second pattern. Also included is asubstrate table for supporting a substrate, a projection system forprojecting the patterned beams onto a target portion of the substrate,and a radiation distribution device that distributes the radiation fromthe illumination system to the patterning means. The radiationdistribution device has a duty cycle during which it sequentiallydirects substantially all of the radiation from the illumination systemto each of a plurality of radiation distribution channels in turn andthe radiation distribution channels provide the beams of radiation tothe patterning means.

By use of such an arrangement, a single radiation source can efficientlyprovide radiation for a plurality of patterning means. Consequently, theapparatus cost is reduced by having a separate illumination system foreach patterning means. Furthermore, use of a single illumination systemreduces the overall size of the apparatus. As a further advantage, byusing a single illumination system, the variation in intensity providedto each patterning means may be less than if each patterning means wassupplied by an independent illumination system. Consequently, theradiation dose provided by different patterning means will be moreconsistent, reducing any effects on the pattern produced on thesubstrate caused by transitions between different regions that have beenilluminated at slightly different intensities.

Such an arrangement may, of course, be scaled up to distribute theradiation from one illumination system to three or more patterningmeans. In a preferred arrangement of each radiation distribution channeldirects radiation to a single one of the patterning means. This approachhas the advantage of each patterned beam having the full intensityradiation of the illumination system, reducing the time required toprovide a given radiation dose.

Alternatively, however, at least one of the radiation distributionchannels may include a beam splitter for dividing the radiation directedinto that radiation distribution channel and distributing it to two ormore patterning means. This arrangement allows a greater number ofpatterning means to be illuminated by a single illumination system.

The radiation distribution device may include a rotatably mountedreflector and a driver for rotating it. The device is arranged in thepath of the beam of radiation from the illumination system such that asthe reflector rotates, the reflection of the beam of radiation changesdirection. This change, in turn, directs the beam to each of theradiation distribution channels. Such an arrangement provides a simpleapparatus for directing substantially all of the radiation from theillumination system to each of the radiation distribution channels inturn, and hence, to each of the patterning means at different times. Theillumination system may include a pulsed radiation source providingpulses of radiation at regular intervals. In this case, the reflectormay be arranged to rotate at a substantially constant speed,synchronized to the rate of the pulsed radiation source. Thesynchronization is such that at each pulse, the reflector is at theangle necessary to reflect the beam of radiation from the illuminationsystem into the required one of the radiation distribution channels.

The radiation distribution device may include a plurality of reflectorsmounted about an axis of rotation (and associated driver) such that asthe plurality of reflectors rotate, each reflector in turn intersectsthe path of the beam of radiation from the illumination system. Duringthe time when each reflector is in the path of the beam of radiationfrom the illumination system, the reflector turns, changing thedirection of the reflected beam of radiation. Therefore, each reflectormay be used to distribute the radiation between the radiationdistribution channels. Again, the illumination system may include apulsed source, providing pulses of radiation at regular intervals. Inthis case, the rotation of the plurality of reflectors may besynchronized to the pulse rate of the radiation source such that, duringeach pulse, one of the reflectors is at the appropriate angle to reflectthe radiation to one of the radiation distribution channels.

The apparatus may be arranged such that the number of reflectors mountedon the radiation distribution device is an integer multiple of thenumber of radiation distribution channels. Each reflector is associatedwith one of the radiation distribution channels. In this case, a pulsedillumination system could be arranged such that, during the time inwhich each reflector intersects the path of the beam of radiation fromthe illumination system, one pulse is emitted. Each reflector may thenbe mounted at an appropriate angle such that during its associatedpulse, the reflector is at the required angle relative to the beam ofradiation from the illumination system. This approach reflects the beamto the associated one of the radiation distribution channels.

In other words, although the radiation distribution device rotates bythe same angle between each pulse of the illumination system, the anglepresented by the reflectors relative to the beam of radiation from theillumination system is different for successive pulses. Consequently,successive pulses of the illumination system are directed to differentradiation channels by merely rotating the radiation distribution deviceat a constant speed, synchronized to the pulse rate of the illuminationsystem.

In an alternative arrangement, the reflectors are mounted on theradiation distribution device such that the radiation distributiondevice rotates through an angle equal to 360 degrees divided by thenumber of reflectors. When rotating in this manner, the angle ofsuccessive reflectors intersecting the beam path at that instant,relative to the beam of radiation from the illumination system, remainsconstant. In other words, were the illumination system arranged toprovide a pulse at those times, the reflected beam from each reflectorwould follow the same path. However, the radiation source may then bearranged to pulse at regular intervals that do not correspond to thetime taken for the time dividing element, rotating at a substantiallyconstant speed, to turn through the angle referred to above.

Accordingly, at each pulse of the illumination system, the beam ofradiation from the illumination system is incident on differentreflectors. And, at successive pulses, the angle of the reflectorrelative to the beam of radiation is different, and the beam isreflected to different radiation distribution channels. The advantage ofthis arrangement is that a simple shaped reflector, for example, onewith a cross-section in the shape of a regular polygon can be used. Incontrast, in the previously discussed arrangement the cross-section ofthe reflector would not be regular.

According to a further aspect of the invention, there is provided alithographic apparatus including an illumination system for supplying aprojection beam of radiation and patterning means serving to impart theprojection beam with a pattern in its cross-section. Second patterningmeans are also included. This second patterning means serves to impart asecond beam of radiation supplied by the illumination system with asecond pattern. Also included are a substrate table for supporting asubstrate and a projection system for projecting the patterned beamsonto a target portion of the substrate.

The lithographic apparatus further includes a radiation distributiondevice that distributes the radiation from the illumination system tothe patterning means. The radiation distribution device includes a beamdivider that divides the beam of radiation from the illumination systeminto a plurality of portions. Each of the plurality of portions isdirected to a distribution channel and the radiation distributionchannels provide the beams of radiation to the patterning means. Thebeam divider comprises a plurality of partially reflective surfacesthrough which the beam of radiation from the illumination system issuccessively directed. Each of the partially reflective surface isassociated with one of the radiation distribution channels andreflecting a portion of the beam of radiation to said radiationdistribution channel.

This arrangement provides an easily controlled means for dividing eachpulse of the illumination system, for example, between the patterningmeans. For example, the proportions of the beam of radiation that arereflected by each of the successive partially reflective surfaces to theassociated radiation distribution channels may be selected such that theintensity of the beam of radiation directed to each of the radiationdistribution channels is the same. Since these proportions aresubstantially time-constant, a variation from a perfectly equaldistribution of the radiation intensity of the beam of radiation fromthe illumination system can be measured and therefore, subsequently, becompensated for.

The beam divider may preferably be formed from a plurality of pieces ofmaterial that is transparent to the radiation generated by theillumination system, for example from quartz or glass. Each piece oftransparent material may have an elongated shape with aradiation-receiving end at one end and the partially reflective surfaceat the other end. The partially reflective surface may be arranged at anangle relative to the direction of the beam of radiation such that partof the beam of radiation is reflected out of the beam divider to theassociated radiation distribution channel and the remainder passesthrough to the next piece of transparent material.

A coating may be applied to the partially reflective surface in order toadjust the proportion of the radiation that is reflected to theradiation distribution channel, as appropriate. The final piece oftransparent material may have a fully reflective surface at the oppositeend to its radiation-receiving end such that all of the remainingradiation from the beam generated by the illumination system is directedto the associated radiation distribution channel.

In any one of the arrangements discussed above, the patterning means maypreferably be an array of individually controllable elements.Accordingly, the array of individually controllable elements may be setto impart to the associated beam of radiation any desired pattern in itscross-section. Therefore, for example, in apparatus according to thefirst aspect of the invention, the use of the radiation distributiondevice has the advantage that while the pattern on one array is beingupdated (i.e., set to a new pattern), the pattern set on another arraymay pattern a beam of radiation which exposes the pattern on thesubstrate.

Additionally, the projection system may include a common element forprojecting all of the patterned beams onto the substrate. In other wordsa single unified projection system may be used. Where arrays ofindividually controllable elements are being used as the patterningmeans with such an arrangement, six to nine arrays of individuallycontrollable elements may preferably be used. Alternatively, theprojection system may include projection system sub-units that eachproject the radiation from one of the patterning means onto a separatetarget portion of the substrate. Accordingly, in such a system, separatecontrol of the projection of each of the patterned beams may beprovided.

Furthermore, because each patterning means has its own projectionsystem, the number of patterning means used is not limited by the sizeof the optical elements that can be manufactured for the projectionsystem. Accordingly a greater number of patterning means may be used. Insuch an arrangement that uses arrays of individually controllableelements, for example, 20 to 30, or even more, arrays of individuallycontrollable elements may be used.

Furthermore, in any one of the arrangements discussed above, theradiation distribution channels may include a liquid light guide, aglass fiber or other fiber optic cable or any optical beam shapingsystem with mirrors and lenses for collecting light from the radiationdistribution system and directing it to one of the patterning means.This may be necessary if, for example, the direction of the radiationfrom the radiation distribution device is not aligned with the opticalaxis of the of the patterning means. Additionally, it may be useful toprovide the necessary separation between each of the patterning means(for example, if the patterning means are arrays of individuallycontrollable elements they may require support services, etc.). Finally,such an arrangement also allows for the illumination system andradiation distribution device to be housed separately from the remainderof the apparatus, if required.

In addition, in any of the arrangements described above, theillumination system may comprise a single radiation source, providing abeam of radiation that is distribution to each of the patterning means.Alternatively, the illumination system may comprise two or moreindependent radiation sources. Each radiation source can generate a beamof radiation. A beam combiner can also be included for combining theseparate beams of radiation to form the beam of radiation from theillumination system which is subsequently distributed between each ofthe patterning means.

This arrangement allows for a beam of greater radiation intensity to begenerated by the illumination system. In addition, radiation sourcesvary in intensity during operation but the variation of the intensity ofthe sum of the beams of radiation generated by independent radiationsources is lower. The beam combiner may include a radiation beamintegrator in order to ensure that, even if the beams of radiationgenerated by each radiation source have different intensities, theintensity distribution the cross-section of the combined beam ofradiation from the illumination system is substantially uniform.

According to yet another aspect of the invention, there is provided adevice manufacturing method including the steps of providing asubstrate, providing a projection beam of radiation using anillumination system, and using patterning means to impart the projectionbeam with a pattern in its cross-section. Next, a second patterningmeans is used to impart a second projection beam of radiation suppliedby the illumination system with a second pattern in its cross-sectionand the patterned beams are projected onto a target portion of thesubstrate. A radiation distribution device is used to distribute theradiation from the illumination system to the patterning means. Theradiation distribution device has a duty cycle during which itsequentially directs substantially all of the radiation from theillumination system to each of a plurality of radiation distributionchannels in turn. The radiation distribution channels provide the beamsof radiation to the patterning means.

Further embodiments, features, and advantages of the present invention,as well as the structure and operation of the various embodiments of thepresent invention are described in detail below with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIG. 2 depicts the arrangement of part of the apparatus according to afirst embodiment of the invention;

FIG. 3 depicts the arrangement of part of the apparatus according to asecond embodiment of the invention;

FIG. 4 depicts part of the apparatus in a variant of the secondembodiment of the invention;

FIG. 5 depicts the arrangement of part of the apparatus according to athird embodiment of the invention;

FIG. 6 depicts the arrangement of part of the apparatus according to asecond aspect of the invention;

FIG. 7 depicts an alternative arrangement of the part of the apparatusshown in FIG. 6; and

FIG. 8 depicts an arrangement of part of the apparatus according to athird aspect of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description of the present invention refers tothe accompanying drawings that illustrate exemplary embodimentsconsistent with this invention. Other embodiments are possible, andmodifications may be made to the embodiments within the spirit and scopeof the invention. Therefore, the detailed description is not meant tolimit the invention. Rather, the scope of the invention is defined bythe appended claims.

It would be apparent to one of skill in the art that the presentinvention, as described below, may be implemented in many differentembodiments of software, hardware, firmware, and/or the entitiesillustrated in the figures. Any actual software code with thespecialized control of hardware to implement the present invention isnot limiting of the present invention. Thus, the operational behavior ofthe present invention will be described with the understanding thatmodifications and variations of the embodiments are possible, given thelevel of detail presented herein.

As a preliminary matter, the term “array of individually controllableelements” as used herein should be broadly interpreted as referring toany means that can be used to endow an incoming radiation beam with apatterned cross-section, so that a desired pattern can be created in atarget portion of the substrate. The terms “light valve” and “SpatialLight Modulator” (SLM) can also be used in this context. Examples ofsuch patterning means are provided below.

A programmable mirror array can comprise a matrix-addressable surfacehaving a viscoelastic control layer and a reflective surface. The basicprinciple behind such an apparatus is that (for example) addressed areasof the reflective surface reflect incident light as diffracted light,whereas unaddressed areas reflect incident light as undiffracted light.Using an appropriate spatial filter, the said undiffracted light can befiltered out of the reflected beam, leaving only the diffracted light toreach the substrate. In this manner, the beam becomes patternedaccording to the addressing pattern of the matrix-addressable surface.It will be appreciated that, as an alternative, the filter may filterout the diffracted light, leaving the undiffracted light to reach thesubstrate.

An array of diffractive optical micro-electro-mechanical systems (MEMS)devices can also be used in a corresponding manner. Each diffractiveoptical MEMS device is comprised of a plurality of reflective ribbonsthat can be deformed relative to one another to form a grating thatreflects incident light as diffracted light. A further alternativeembodiment of a programmable mirror array employs a matrix arrangementof tiny mirrors, each of which can be individually tilted about an axisby applying a suitable localized electric field, or by employingpiezoelectric actuation means. Once again, the mirrors arematrix-addressable, such that addressed mirrors will reflect an incomingradiation beam in a different direction to unaddressed mirrors. In thismanner, the reflected beam is patterned according to the addressingpattern of the matrix-addressable mirrors. The required matrixaddressing can be performed using suitable electronic means.

In both of the situations described hereabove, the array of individuallycontrollable elements can comprise one or more programmable mirrorarrays. More information on mirror arrays as here referred to can begleaned, for example, from U.S. Pat. No. 5,296,891 and U.S. Pat. No.5,523,193, and PCT patent applications WO 98/38597 and WO 98/33096,which are incorporated herein by reference.

An example of a programmable LCD array of such a construction is givenin U.S. Pat. No. 5,229,872, which is incorporated herein by reference.

It should be appreciated that where pre-biasing of features, opticalproximity correction features, phase variation techniques and multipleexposure techniques are used, for example, the pattern “displayed” onthe array of individually controllable elements may differ substantiallyfrom the pattern eventually transferred to a layer of or on thesubstrate. Similarly, the pattern eventually generated on the substratemay not correspond to the pattern formed at any one instant on the arrayof individually controllable elements. This may be the case in anarrangement in which the eventual pattern formed on each part of thesubstrate is built up over a given period of time or a given number ofexposures during which the pattern on the array of individuallycontrollable elements and/or the relative position of the substratechanges.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications. One such other application is the manufacture ofintegrated optical systems, guidance and detection patterns for magneticdomain memories, flat panel displays, thin-film magnetic heads, etc. Theskilled artisan will appreciate that, in the context of such alternativeapplications, any use of the terms “wafer” or “die” herein may beconsidered as synonymous with the more general terms “substrate” or“target portion”, respectively.

The term substrate referred to herein may be processed, before or afterexposure, in for example a track (a tool that typically applies a layerof resist to a substrate and develops the exposed resist) or a metrologyor inspection tool. Where applicable, the disclosure herein may beapplied to such and other substrate processing tools. Further, thesubstrate may be processed more than once, for example in order tocreate a multilayer IC, so that the term substrate used herein may alsorefer to a substrate that already contains multiple processed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.,having a wavelength of 365, 248, 193, 157 or 126 nm) and extremeultra-violet (EUV) radiation (e.g., having a wavelength in the range of520 nm), as well as particle beams, such as ion beams or electron beams.

The term “projection system” used herein should be broadly interpretedas encompassing various types of projection system, including refractiveoptical systems, reflective optical systems, and catadioptric opticalsystems, as appropriate for example for the exposure radiation beingused, or for other factors such as the use of an immersion fluid or theuse of a vacuum. Any use of the term “lens” herein may be considered assynonymous with the more general term “projection system.”

The illumination system may also encompass various types of opticalcomponents, including refractive, reflective, and catadioptric opticalcomponents for directing, shaping, or controlling the projection beam ofradiation, and such components may also be referred to below,collectively or singularly, as a “lens.”

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein the substrateis immersed in a liquid having a relatively high refractive index, e.g.,water, so as to fill a space between the final element of the projectionsystem and the substrate. Immersion liquids may also be applied to otherspaces in the lithographic apparatus, for example, between the mask andthe first element of the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems.

FIG. 1 schematically depicts a lithographic projection apparatusaccording to a particular embodiment of the invention. The apparatusincludes an illumination system (illuminator) IL for providing aprojection beam PB of radiation (e.g., UV radiation) and an array ofindividually controllable elements PPM (e.g., a programmable mirrorarray) for applying a pattern to the projection beam. In general theposition of the array of individually controllable elements will befixed relative to item PL. However, it may instead be connected to apositioning means for accurately positioning it with respect to the itemPL. Also included is a substrate table (e.g., a wafer table) WT forsupporting a substrate (e.g., a resist-coated wafer) W, and connected topositioning means PW for accurately positioning the substrate withrespect to item PL. Finally, a projection system (“lens”) PL is includedfor imaging a pattern imparted to the projection beam PB by the array ofindividually controllable elements PPM onto a target portion C (e.g.,comprising one or more dies) of the substrate W.

The projection system may image the array of individually controllableelements onto the substrate. Alternatively, the projection system mayimage secondary sources for which the elements of the array ofindividually controllable elements act as shutters. The projectionsystem may also comprise a micro lens array (known as an MLA), e.g., toform the secondary sources and to image microspots onto the substrate.

The apparatus depicted here is of a reflective type (i.e., has areflective array of individually controllable elements). However, ingeneral, it may also be of a transmissive type, for example (i.e., witha transmissive array of individually controllable elements).

The illuminator IL receives a beam of radiation from a radiation sourceSO. The source and the lithographic apparatus may be separate entities,for example when the source is an excimer laser. In such cases, thesource is not considered to form part of the lithographic apparatus andthe radiation beam is passed from the source SO to the illuminator ILwith the aid of a beam delivery system BD comprising for examplesuitable directing mirrors and/or a beam expander. In other cases thesource may be integral part of the apparatus, for example when thesource is a mercury lamp. The source SO and the illuminator IL, togetherwith the beam delivery system BD if required, may be referred to as aradiation system.

The illuminator IL may comprise adjusting means AM for adjusting theangular intensity distribution of the beam. Generally, at least theouter and/or inner radial extent (commonly referred to as cy-outer anda-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator ILgenerally comprises various other components, such as an integrator INand a condenser CO. The illuminator provides a conditioned beam ofradiation, referred to as the projection beam PB, having a desireduniformity and intensity distribution in its cross-section.

The beam PB subsequently intercepts the array of individuallycontrollable elements PPM. Having been reflected by the array ofindividually controllable elements PPM, the beam PB passes through theprojection system PL, which focuses the beam PB onto a target portion Cof the substrate W. With the aid of the positioning means PW (andinterferometric measuring means IF), the substrate table WT can be movedaccurately (e.g., so as to position different target portions C in thepath of the beam PB).

Where used, the positioning means for the array of individuallycontrollable elements can be used to accurately correct the position ofthe array of individually controllable elements PPM with respect to thepath of the beam PB (e.g., during a scan). In general, movement of theobject table WT is realized with the aid of a long-stroke module (coursepositioning) and a short-stroke module (fine positioning), which are notexplicitly depicted in FIG. 1. A similar system may also be used toposition the array of individually controllable elements.

It will be appreciated that the projection beam mayalternatively/additionally be moveable while the object table and/or thearray of individually controllable elements may have a fixed position toprovide the required relative movement. As a further alternative, thatmay be especially applicable in the manufacture of flat panel displays,the position of the substrate table and the projection system maybefixed and the substrate may be arranged to be moved relative to thesubstrate table. For example, the substrate table may be provided with asystem for scanning the substrate across it at a substantially constantvelocity.

Although the lithography apparatus according to the invention is hereindescribed as being for exposing a resist on a substrate, it will beappreciated that the invention is not limited to this use and theapparatus may be used to project a patterned projection beam for use inresistless lithography.

The depicted apparatus can be used in four preferred modes. In a stepmode, the array of individually controllable elements imparts an entirepattern to the projection beam, which is projected onto a target portionC in one go (i.e., a single static exposure). The substrate table WT isthen shifted in the X and/or Y direction so that a different targetportion C can be exposed. In step mode, the maximum size of the exposurefield limits the size of the target portion C imaged in a single staticexposure.

In a scan mode, the array of individually controllable elements ismovable in a given direction (the so-called “scan direction”, e.g., theY direction) with a speed v, so that the projection beam PB is caused toscan over the array of individually controllable elements; concurrently,the substrate table WT is simultaneously moved in the same or oppositedirection at a speed V=Mv, in which M is the magnification of the lensPL. In scan mode, the maximum size of the exposure field limits thewidth (in the non-scanning direction) of the target portion in a singledynamic exposure, whereas the length of the scanning motion determinesthe height (in the scanning direction) of the target portion.

In a pulse mode, the array of individually controllable elements is keptessentially stationary and the entire pattern is projected onto a targetportion C of the substrate using a pulsed radiation source. Thesubstrate table WT is moved with an essentially constant speed such thatthe projection beam PB is caused to scan a line across the substrate W.The pattern on the array of individually controllable elements isupdated as required between pulses of the radiation system and thepulses are timed such that successive target portions C are exposed atthe required locations on the substrate. Consequently, the projectionbeam can scan across the substrate W to expose the complete pattern fora strip of the substrate. The process is repeated until the completesubstrate has been exposed line by line.

A continuous scan mode is also provided. The continuous scan mode isessentially the same as pulse mode except that a substantially constantradiation source is used and the pattern on the array of individuallycontrollable elements is updated as the projection beam scans across thesubstrate and exposes it. Combinations and/or variations on the abovedescribed modes of use or entirely different modes of use may also beemployed.

FIG. 2 schematically represents part of a lithographic projectionapparatus. An illumination system 5 produces a beam of radiation 6 whichis distributed by a radiation distribution system 7 to a plurality oflight engines 8 which pattern the radiation beam and project it onto asubstrate 9. Each light engine 8 includes an array of individuallycontrollable elements for patterning a beam of radiation according to adesired pattern and a projection system for projecting the patternedbeam onto the substrate. The light engines 8 may also include additionalelements for preparing the beam of radiation prior to it being incidenton the array of individually controllable elements.

For example, included may be the components to compensate for the angleat which the light engines 8 receive the radiation from the radiationdistribution system 7. It will be appreciated that instead of havingseparate projection systems, a plurality of the light engines may bearranged with a common projection system for simultaneously projectingthe patterned beams generated by the arrays of individually controllableelements onto the substrate. Furthermore, it will also be realized thatthat the present invention is not limited to use with arrays ofindividually controllable elements for patterning the beams ofradiation. In general, any patterning means for imparting a pattern tothe cross-section of the beam of radiation may be used in place of thearrays of individually controllable elements described.

As shown in FIG. 2, the radiation distribution system 7 is comprised ofa reflector which is rotatably mounted in the path of the beam ofradiation 6 from the illumination system 5. At different angles ofrotation, the reflector reflects the beam of radiation 6 from theillumination system to different light engines 8. Accordingly, as adriver (not shown) rotates the reflector, the radiation from theillumination system 5 is directed to each of the light engines 8 inturn. The reflector may be arranged to rotate reciprocally such that theradiation is directed backwards and forwards along the line of lightengines 8. In order to achieve this, the reflector may to actuated bypiezo-electric actuators, by electro-static actuators, by Lorentzactuators or by any other appropriate means.

Alternatively, the reflector may be arranged to rotate at a constantspeed around an axis such that the radiation is directed repeatedlyalong the row of light engines 8. The rotating reflector of theradiation distribution system 7 may be a planar element as shown in FIG.2. In such an arrangement, both sides of the planar element may comprisereflective surfaces such that for each half turn of the element, theradiation is distributed to each of the light engines 8 in turn.

It will be appreciated that, although FIG. 2 shows the light engines 8arranged in a single row, in practice the light engines may be arrangedin any fashion, for example in two or more rows, as convenient.Therefore the radiation from the illumination system 5 will need to bedistributed in direction perpendicular to the plane of FIG. 2 as well.This may be arranged by enabling the reflector of the radiationdistribution system 7 to not only rotate about an axis perpendicular tothe plane of FIG. 2 but also, perhaps by a more limited extent, about asecond, orthogonal, axis. Alternatively, the reflector may distributethe radiation about a single axis as shown in FIG. 2 and distributionelements may be provided to carry the radiation to the light engines 8.FIG. 6 illustrates this.

As shown, in FIG. 6, the light engines 8 are arranged in two rows. Aradiation distribution unit 45 distributes the radiation from anillumination system 5 to a plurality of radiation distribution channels46. Distribution elements 47 then carry the radiation from thedistribution channels 46 to the light engines 8. Preferably thedistribution elements also ensure that the radiation reaches the lightengines 8 aligned in a direction parallel to their optical axes.

FIG. 7 illustrates a similar arrangement but in which the radiationdistribution unit 50 is arranged with parallel output beams of radiationinstead of the radial arrangement shown in FIG. 6. The radiationdistribution elements 47 may be formed from liquid light guides, glassfibers or other fiber optic cables, as appropriate. Consequently, anyconfiguration of outputs from a radiation distribution unit can be usedin conjunction with any layout of light engines 8. Furthermore, theillumination system and the radiation distribution system may becontained in a separate housing than that of the remainder of thelithographic apparatus.

If required, the radiation distribution system 7 may also directradiation to a radiation distribution channel that is not connected toone of the light engines 8. For example, one radiation distributionchannel may be connected to a sensor, for example to measure theradiation intensity level. This may be advantageous because manyradiation sources vary in intensity over time. However, the variation ofthe intensity level is typically gradual. Therefore it may only benecessary to monitor the intensity of the radiation periodically.

As a further variation, the radiation from the radiation distributionsystem 7 may not be directed to each light engine directly. Instead,each radiation distribution channel may include one or more radiationbeam splitters for dividing the radiation directed to that channel atany given instant and distributing it to two or more light engines.Similarly, each light engine may include one or more arrays ofindividually controllable elements that are illuminated in the samefield and/or share a common projection system.

In a preferred arrangement, the illumination system 5 produces pulses ofradiation at regular intervals, namely includes a pulsed radiationsource. In this case, the rotation of the radiation distribution system7 is synchronized to the pulse rate of the illumination system 5. Forexample, the synchronization may be such that during a single rotationof the radiation distribution system 7 (or half-turn if, for example,the element is rotating at a continuous speed and is double-sided), theillumination system provides pulses of radiation at each point that thereflector is at the required angle to reflect radiation to each of theradiation distribution channels (or directly to the light engines 8 asshown in FIG. 2).

Alternatively, for example, the synchronization may be such that duringeach turn, the illumination system provides a pulse for only one of theradiation distribution channels or provides radiation to alternatechannels in each rotation. It will be appreciated that other duty cyclescan also be considered. For example, if, as discussed above, the lightengines 8 are arranged in more than one row, the synchronization may besuch that in each rotation of the radiation distribution system 7 aboutan axis perpendicular to the plane of FIG. 2, the illumination systemprovides pulses of radiation for each of the light engines 8 in a singlerow. Subsequently, the radiation distribution element is moved about thesecond axis and the next rotation of the radiation distribution system 7delivers radiation to another row of light engines.

Consequently, as described above, the radiation distribution system 7has a duty cycle in which the radiation from the illumination system isdistributed to a plurality of radiation distribution channels in turn.Each of the radiation distribution channels subsequently directs theradiation to one or more light engines containing arrays of individuallycontrollable elements for patterning the beams of radiation. Therefore,while one array of individually controllable elements is beingilluminated and the consequent patterned beam of radiation is beingprojected onto the substrate, other arrays of individually controllableelements may be having the next pattern being set.

This approach is useful because a pulsed radiation source may be able toprovide pulses of radiation faster than the arrays of individuallycontrollable elements can be set to new patterns. Therefore, bydistributing the pulses of radiation from a single illumination systemto a plurality of arrays of individually controllable elements, theillumination system can be used more efficiently and the size and costof the apparatus is less than if independent illumination systems wererequired for each light engine, for example.

It will be appreciated that, in addition to synchronizing the radiationdistribution system to the pulsed radiation source, it is necessary tosynchronize both of these with the updating of the pattern on each ofthe arrays of individually controllable elements.

The illumination system may comprise a single radiation source. However,it may also comprise two or more radiation sources in order to providesufficient radiation intensity in each pulse of the illumination system.For example, this may be necessary to keep the exposure time required toachieve a given radiation dose below a set level. The use of multipleradiation sources may also be used to improve the consistency of theradiation intensity over time, thereby improving the control of theradiation dose received in each area illuminated on the substrate.

Consequently, for a given number of light engines that are being used toproject patterned beams of radiation onto the substrate, the apparatusmay include an illumination system with the same number of radiationsources as there are light engines, a system for combining the beams ofradiation produced by each of the sources and, subsequently, a radiationdistribution system as described above or below, for re-distributing theradiation between each of the light engines. It will be appreciatedthat, in general, any number of sources may be used in conjunction withany number of light engines. A further advantage is that even if theintensity of the combined beam of radiation does still vary to someextent, variation between the intensities of the beams of radiationbeing patterned and projected onto the substrate at a given time will bereduced in comparison with an apparatus using a plurality of independentilluminations system associated with each light engine.

FIG. 8 represents a simple arrangement for combining beams of radiationfrom two sources. A combining unit 60 comprises two radiation sources61, 62 producing beams of radiation 63, 64, respectively. The beams ofradiation are incident on reflectors 65, 66 to produce a combined beam67. Preferably, the combined beam 67 passes through an integrator 68,which is used to improve the uniformity of the intensity distributionacross the combined beam 67. An integrated beam 69 may then pass througha condenser 70 for reducing the size of the combined beam and,correspondingly, increasing its intensity. It will be appreciated,however, that other beam combining units may also be used with thepresent invention.

In addition to the above, the illumination system may include a shutter.This may be used to improve the control of the pulses when using apulsed illumination system. For example, it may be used to provide arequired radiation intensity profile over time.

FIG. 3 schematically represents a second embodiment according to theinvention. In this case the planar reflector of the radiationdistribution element is replaced by a radiation distribution element 15with a plurality of reflective surfaces 16 mounted around an axis 17. Asthe radiation distribution element 15 rotates, each reflective surface16 in turn intersects the beam of radiation 6 from the illuminationsystem 5. During each such pass, the angle that each reflective surface16 presents to the beam of radiation 6 from the illumination system 5changes. Consequently, a beam of radiation reflected from the reflectivesurface 16 also changes direction during that time. Therefore, in amanner corresponding to that described above in relation to the firstembodiment, each reflective surface 16 can be used to distribute theradiation between each of the radiation distribution channels in turnduring the time that it intersects the beam of radiation 6 from theillumination system 5.

For example, an apparatus using a pulsed illumination system may besynchronized such that during the time in which each reflective surface16 intersects the beam of radiation 6, the illumination system 5generates a plurality of pulses, each corresponding to, and beingreflected to, one of the plurality of radiation distribution channels.With such an arrangement, the reflective surfaces 16 may be uniformlydistributed around the axis of rotation 17 of the radiation distributionelement 15 such that, in cross-section, the combination of thereflective surface 16 forms a regular polygon, as shown in FIG. 3.

In an alternative arrangement according to this embodiment, theapparatus may be configured such that a pulsed illumination system 5produces only a single pulse of radiation during the time that eachreflective surface 16 intersects the line of the beam of radiation 6from the illumination system 5. In this arrangement, the radiationdistribution element 15 may rotate at a constant speed that does notmatch the pulse rate of the illumination system 5. For example, in sucha configuration, during any given number of pulses of the illuminationsystem, the radiation distribution element 15 does not complete a singlerevolution. Therefore, each pulse of the illumination system 5 isincident on different reflective surfaces 16 of the radiationdistribution element 15 and the angle presented by the successivereflective faces 16 of the radiation distribution element 15 atsuccessive pulses is different and the reflected beam is directed todifferent radiation distribution channels in successive pulses.

FIG. 4 is an illustration of a radiation distribution element 20 used ina further variant of the second embodiment of the invention. In thiscase, the reflective surfaces 22, 23, 24, 25, 26, 27 are not arranged toform a regular polygon. Instead, as schematically represented, eachreflector is arranged at a different angle, compared to the otherreflective surfaces, to the position that the reflective surface wouldhave been at were they to form a regular polygon (such a regular polygon21 is shown in broken lines for comparison).

For clarity, the reflective surfaces have not been shown joined to oneanother. However, it will be appreciated, that in practice, they may beso joined, for example if the radiation distribution element 20 weremade from a solid piece of material. With such an arrangement, theradiation distribution element 20 can be arranged to rotate at the samespeed as (or an integer multiple of) the pulse rate of the illuminationsystem 5. In particular, although between each pulse of the illuminationsystem the radiation distribution element rotates by a constant amount,the angle presented by each of the reflectors in turn to the beam ofradiation 6 from the illumination system is different and therefore thebeam is reflected to each of the radiation distribution channels inturn.

It will be appreciated that a combination of the arrangements describedabove may be used together. For example, a radiation distributionelement similar to that shown in FIG. 4 may be used, but in which one ormore of the reflective surfaces is also rotated about an axis within theplane of FIG. 4. Consequently, radiation may be reflected to radiationdistribution channels (or light engines) arranged in two or moreadjacent rows. Additionally or alternatively, an apparatus using aradiation distribution element 20 as shown in FIG. 4 may be arrangedsuch that, during the time that each reflective surface is in line withthe beam of radiation from the illumination system, two or more pulsesof radiation are incident on each reflective surface as the radiationdistribution element 20 rotates. Therefore the beam of radiation isdistributed to two or more corresponding radiation distribution channelsin each such portion of the duty cycle of the radiation distributionelement.

Instead of using reflectors with a flat surface, one or more of thereflectors may have, for example a curved surface. In this case, theprofile of the reflector may be chosen such that, over a given timeduration during which the reflector rotates, although the angle of thereflector as a whole relative to the beam of radiation from theillumination system changes, the angle of the beam of radiation relativeto each point on the surface of the reflector on which it is incidentremains constant. Accordingly, during that time duration, the directionof the reflected beam of radiation also remains constant, for exampletowards the appropriate one of the radiation distribution channels.

If a pulsed radiation source is used, as discussed above, the profilesof the reflectors (or a portion of the reflectors) may be chosen suchthat during the entirety of a pulse of radiation from the illuminationsystem the radiation is directed towards one of the radiationdistribution channels, even if the reflector as a whole rotates by asignificant amount relative to the beam of radiation from theillumination system during the pulse.

Alternatively, if a non-pulsed illumination system is being used, theprofile of each reflector may be such that for the duration of the timethat the radiation beam is incident on a given reflector (as theradiation distribution element rotates), the radiation beam is reflectedto the associated radiation distribution channel. As the radiationdistribution element rotates further, the beam of radiation from theillumination system becomes incident on another reflector, with aprofile such that the beam of radiation is then directed to anotherradiation distribution channel.

FIG. 5 represents a third embodiment of the present invention. Providedis a radiation distribution system that can simultaneously provideradiation to two or more radiation distribution channels (or lightengines 8). In particular, where a pulsed illumination system is beingused, the radiation distribution element 30 can divide each pulsebetween the radiation distribution channels. Furthermore, thearrangement shown can be used as a beam divider within one of theradiation distribution channels, as described above.

As shown in FIG. 5, a radiation distribution element 30 is comprised ofa plurality of sections 31, 32, 33, 34, 35, 36, each associated with aradiation distribution channel or light engine 8. Each section iscomprised of a material which is substantially transmissive to theradiation used and, may be formed from rods of glass or quartz, inparticular. It will be appreciated, however, that the cross-sectionalshape of these sections may be any convenient shape.

The first section 31 has a first end 31 b that receives a beam ofradiation 6 from the illumination system 5. At the other end, the firstsection has a partially reflective surface 31 a arranged at an angle tothe beam of radiation 6. A portion of the beam of radiation 6 isreflected out of the radiation distribution element 30 to an associatedradiation distribution channel or light engine 8. The remainder of thebeam of radiation passes through the partially reflective surface intothe second section 32 of the radiation distribution element 30. Asshown, the second section is conveniently shaped such that the end thatreceives the radiation has a shape corresponding to the partiallyreflective surface 31 a of the first section. However, this need not bethe case.

The second section, 32 also has a partially reflective surface at theopposite end, similar to that of the first section that in turn reflectsa portion of the remaining beam of radiation out of the radiationdistribution element 30 and permits the remainder of the radiation topass into the third section. This is repeated as necessary until thefinal section, in the example shown the sixth section 36, which has afully reflective surface 36 a for reflecting the remaining portion ofthe beam of radiation 6 from the illumination system 5 into theassociated radiation distribution channel or light engine 8.

By suitable arrangement of the partially reflective surfaces, theradiation distribution element 30 can be arranged to divide the beam ofradiation 6 from the illumination system 5 into a plurality of beams ineach radiation distribution channel with equal intensity. For example,different coatings may be used on each of the partially reflectivesurfaces. Suitable materials for such coatings include fluorides such ascryolite. Furthermore, the proportions of the beam of radiation 6 thatwill be directed to each of the radiation distribution channels willremain constant over time. Therefore if the distribution of theradiation intensity is not perfectly even, the relative intensities ineach of the radiation distribution channels may be measured andcompensated for appropriately in the remainder of the apparatus.

It will be appreciated that, as with the previous embodiments, theradiation distribution channels or light engines 8 may be arranged otherthan in a single row as shown in FIG. 5. In this case radiationdistribution elements as shown in FIG. 7 may be used to direct theradiation accordingly or variation of the arrangement of the radiationdistribution element 30 shown in FIG. 5 may be used. For example, fullreflectors may be arranged between one or more of the section of theradiation distribution element 30 such that the individual sections ofthe radiation distribution element 30 need not be aligned.

Similarly, alternative configurations of the sections 31, 32, 33, 34,35, 36, may also be used. For example, the partially reflective surfacesof alternate sections may be angled in opposite directions, reflectingradiation out of the opposite side of the radiation distribution elementto the first partially reflective surfaces. Plane reflectors may then bearranged to reflect all of the portions of the beam of radiation through90 degrees (for example, into the plane of FIG. 5), generating twoparallel rows of beams of radiation that may be projected directly totwo rows of light engines.

CONCLUSION

The present invention has been described above with the aid offunctional building blocks illustrating the performance of specifiedfunctions and relationships thereof. The boundaries of these functionalbuilding blocks have been arbitrarily defined herein for the convenienceof the description. Alternate boundaries can be defined so long as thespecified functions and relationships thereof are appropriatelyperformed.

Any such alternate boundaries are thus within the scope and spirit ofthe claimed invention. One skilled in the art will recognize that thesefunctional building blocks can be implemented by analog and/or digitalcircuits, discrete components, application specific integrated circuits,firmware, processors executing appropriate software and the like or anycombination thereof. Thus, the breadth and scope of the presentinvention should not be limited by any of the above-described exemplaryembodiments, but should be defined only in accordance with the followingclaims and their equivalents.

1. A lithographic apparatus, comprising: an illumination system thatsupplies a projection beam of radiation; patterning means for impartingthe projection beam with a pattern in its cross-section; secondpatterning means for imparting a second beam of radiation supplied bythe illumination system with a second pattern; a substrate table thatsupports a substrate; a projection system that projects the patternedbeams onto a target portion of the substrate; and a radiationdistribution device that distributes the radiation from the illuminationsystem to the patterning means; wherein the radiation distributiondevice has a duty cycle during which it sequentially directssubstantially all of the radiation from the illumination system to eachof a plurality of radiation distribution channels in turn, and whereinthe radiation distribution channels provide the beams of radiation tothe patterning means.
 1. (canceled)
 2. A system, comprising: anilluminator configured to produce a beam of radiation; a radiationdistribution device configured to receive the beam of radiation andproduce at least first and second beams therefrom, the radiationdistribution device configured to sequentially direct the beams ofradiation from the illuminator to corresponding distribution paths toform the at least first and second beams; a first patterning deviceconfigured to pattern the first beam with a first pattern; a secondpatterning device configured to pattern the second beam with a secondpattern; and a projection system configured to project the first andsecond patterned beams onto a target portion of a substrate.
 3. Thesystem of claim 2, further comprising: a third patterning device,wherein the radiation distribution device is configured to produce athird beam and the third patterning device is configured to pattern thethird beam with a third pattern.
 4. The system of claim 2, wherein atleast one of the radiation distribution paths includes a beam splitterconfigured to divide the beam of radiation and direct the divided beamto the first and second patterning devices.
 5. The system of claim 2,wherein: the radiation distribution device comprises a rotatingreflector; and the reflector is located in a path of the beam ofradiation from the illuminator and arranged such that as the reflectorrotates a reflected beam of radiation changes direction, so that thereflected beam is distributed to each of the radiation distributionpaths, in turn.
 6. The system of claim 5, wherein: the illuminator isarranged to provide pulses of the beam of radiation at substantiallyregular intervals; and the reflector rotates at a substantially constantspeed, synchronized to a pulse rate of the illuminator, such that duringeach of the pulses the reflected beam of radiation is reflected to oneof the radiation distribution paths.
 7. The system of claim 2, furthercomprising: wherein the radiation distribution device comprises aplurality of reflectors rotatably coupled about an axis such that, asthe radiation distribution device rotates, each reflector in turn movesinto the path of the beam of radiation for a given time as the beam ofradiation travels from the illuminator, during which time the reflectedbeam of radiation changes direction, distributing the reflected beam toeach of the radiation distribution paths, in turn.
 8. The system ofclaim 7, wherein the illuminator is arranged to provide pulses of thebeam of radiation at substantially regular intervals, wherein theplurality of reflectors rotate at a substantially constant speed,synchronized to a pulse rate of the illuminator, such that during eachof the pulses the reflected beam of radiation is reflected to one of theradiation distribution paths by a respective one of the reflectors. 9.The system of claim 8, wherein: during successive one of the pulses, thebeam of radiation is incident on different one of the reflectors of theplurality of reflectors; and respective ones of the reflectors areassociated with respective ones of the radiation distribution paths andarranged such that, during a given one of the pulses, the respective oneof the reflectors is at an angle relative to the beam of radiation fromthe illuminator, such that the beam is reflected to the associated oneof the radiation distribution channels.
 10. The system of claim 8,wherein the plurality of reflectors rotate such that successive pulsesof radiation are incident on different reflectors and, at successivepulses of the illumination system that are incident on each reflector,the reflector is at different angles relative to the beam of radiationsuch that the radiation is directed to different radiation distributionchannels.
 11. The system of claim 2, wherein the projection systemcomprises first and second projection system sub-units configured toindependently project the first and second patterned beams of radiationonto first and second ones of the target portion of the substrate. 12.The system of claim 2, wherein a radiation distribution path includes aliquid light guide and an optical device configured to collect the firstand second beams and direct the first and second beams to correspondingones of the first and second patterning devices.
 13. The system of claim2, wherein the illuminator comprises: first and second radiation sourcesconfigured to provide respective first and second beams of radiation;and a beam combiner configured to combine the first and second beams ofradiation to form the beam of radiation provided by the illuminator. 14.The system of claim 13, wherein the beam combiner comprises a radiationbeam integrator configured to produce substantially uniform intensity inthe beam of radiation formed from combining the first and second beamsof radiation.
 15. A lithographic apparatus, comprising: an illuminationsystem configured to supply first and second beams of radiation; a firstpatterning device configured to pattern the first beam of radiation witha first pattern; a second patterning device configured to pattern thesecond beam of radiation with a second pattern; a projection system thatprojects the patterned beams onto a target portion of the substrate; anda radiation distribution device that distributes the radiation from theillumination system to the first and second patterning devices, whereinthe radiation distribution device includes an optical device thatdivides the beam of radiation from the illumination system into thefirst and second beams, each of which is directed to a distributionpath, and wherein the radiation distribution paths provide the first andsecond beams of radiation to respective ones of the first and secondpatterning devices and the optical device comprises a plurality ofpartially reflective surfaces through which the first and second beamsof radiation from the illumination system are successively directed inaccordance with a distribution device duty cycle, each partiallyreflective surface associated with one of the radiation distributionpaths and reflecting a portion of the first and second beams ofradiation to the radiation distribution path.
 16. The system of claim15, wherein proportions of the beam of radiation from the illuminationsystem which are reflected by each of the successive partiallyreflective surfaces to the associated radiation distribution paths arearranged such that substantially equal proportions of intensity of thebeam of radiation from the illumination system are directed to each ofthe radiation distribution paths.
 17. The system of claim 15, whereineach of the partially reflective surfaces is arranged on one end of asection of material that is substantially transparent to the radiation,at an angle to the beam of radiation from the illumination system suchthat a proportion of the radiation incident on each of the partiallyreflective surfaces is deflected to the associated radiationdistribution paths and the remainder passes through to a next section oftransparent material.
 18. The system of claim 17, wherein a finalsection of transparent material comprises a fully reflective surfacethat reflects substantially all of the radiation directed into thesection to one of the radiation distribution paths.
 19. The system ofclaim 15, wherein each of the radiation distribution channels directsthe radiation to a one of the first and second patterning devices. 20.The system of claim 15, wherein the projection system includes at leastfirst and second projection system sub-units that independently projectsthe first and second patterned beams of radiation onto separate targetportions of the substrate.
 21. The system of claim 15, wherein at leastone radiation distribution path includes a liquid light guide and anoptical device configured to collect light from the radiationdistribution system and directing it to one of the first or secondpatterning device.
 22. The system of claim 15, wherein the illuminationsystem comprises at least two radiation sources, each providing arespective one of the first and second beams of radiation, and a beamcombiner that combines the source beams of radiation to form the beam ofradiation provided by the illumination system.
 23. The system of claim22, wherein the beam combiner includes a radiation beam integrator thatensures substantially uniform intensity in the beam of radiation formedfrom combining the source beams of radiation even if the source beamshave different intensities to each other.
 24. A device manufacturingmethod, comprising: pattering a first beam of radiation with a firstpattern; patterning a second beam of radiation with a second pattern;projecting the first and second patterned beams onto a target portion ofa substrate; and using a radiation distribution device to distribute theradiation from the illumination system to the first and secondpatterning devices, wherein the radiation distribution device has a dutycycle during which it sequentially directs substantially all of theradiation from the illumination system to each of a plurality ofradiation distribution paths in turn, wherein the radiation distributionpaths provide the beams of radiation to respective ones of the first andsecond patterning devices.